Cracked Stem

Peter Verdesi said:

I had an interesting thing happened to me today. I was on the last hill (a rather steep bugger) of a 60 mile ride and the stem on my Cannondale Multisport 4000 cracked along the weld (I've attached a photo). At first I thought something was loose but then, after a closer look, I found the failure of the weld. I've never crashed the bike so I was very surprised that it happened. I live in the northeast so it's pretty hilly and I've been known to put some torque on the bars when I'm climbing but should this happen? I guess I'm pretty lucky it did not totally fail going down one of the steep hills. Anyone with a similar problem or comments? Thanks.

Wow, thats a sobering picture. Aluminum does fatigue but I would suspect a manufacturing defect like incomplete weld penetration.

Also do you live near the coast or store the bike outside, salt air and moisture can accelerate something called stress corrosion cracking.

Are you a monstrously strong rider, do you climb out of the saddle torqueing on the bars a lot?

Sheesh, scary photo!

Mr_Potatohead said:

Wow, thats a sobering picture. Aluminum does fatigue but I would suspect a manufacturing defect like incomplete weld penetration.

Also do you live near the coast or store the bike outside, salt air and moisture can accelerate something called stress corrosion cracking.

Are you a monstrously strong rider, do you climb out of the saddle torqueing on the bars a lot?

Sheesh, scary photo!

To answer your question about living near the coast; I'm about 70 miles north of New York City so I'm no where near the ocean. In terms of being a monstrously strong rider, I guess I'm a bit bigger then alot of riders (185@6'0"😉. I think alot of what happened is a result of climbing hills in my area (some of which go for several miles at a decent grade). I know that I pull on the bars pretty heavy. I'm just glad it went slowly instead of all at once (face plant). Thanks for the reply.

Peter Verdesi said:

...In terms of being a monstrously strong rider, I guess I'm a bit bigger then alot of riders (185@6'0"😉. I think alot of what happened is a result of climbing hills in my area (some of which go for several miles at a decent grade). I know that I pull on the bars pretty heavy...

Short of trying to tear the bike apart with (2x) D11N Bulldozers, your stem should not have this failure. Components like these are meant to have a large safety factor. A weld failure can be caused by incorrect tempering and / or preparation procedures (which is pretty scarey when it happens in a mass-produced machine-welded item like this). Take plenty of photo's, make contact with the manufacturer for return and replacement, getting written correspondence from them on their acknowledgment of the problem. There may be a bunch of other handlebar stems out there ready to let go and the riders may not get off as fortunately as you did.

thats an interesting picture. Im an engineering student and would have expected failure from fatigue on the top of the stem at the weld. since it looks like the crack started at the bottom under/inside the weld I would think it is most likely manufacturing error or an impurity in the material. To tell for sure you would have to have a trained engineer look at the point of fracture under a microscope. You can tell how it broke from grain patterns ect. A picture is not detailed enough. As for thinking your torquing the bars heavily, it wouldnt fail like that without a defect in the material. the fatigue life for aluminium goes into the millions or tens of millions of cycles before failure. That would be a lot of bike riding.

spacefuzz said:

thats an interesting picture. Im an engineering student and would have expected failure from fatigue on the top of the stem at the weld. since it looks like the crack started at the bottom under/inside the weld I would think it is most likely manufacturing error or an impurity in the material. To tell for sure you would have to have a trained engineer look at the point of fracture under a microscope. You can tell how it broke from grain patterns ect. A picture is not detailed enough. As for thinking your torquing the bars heavily, it wouldnt fail like that without a defect in the material. the fatigue life for aluminium goes into the millions or tens of millions of cycles before failure. That would be a lot of bike riding.

Fatigue life is a function of stress amplitude. There's no absolute number of cycles for a given material. The only thing a microscope would tell you about the fracture is how many cycles there were between crack initiation and failure. I'd be more interested in slicing it to see the weld penetration. The heat affected zone on a weld like that usually extends out pretty far. A crack initiated near a good weld will usually continue to propagate along the side of the weld rather than up the middle. The crack in that picture points toward a shallow weld.

spacefuzz said:

...would have expected failure from fatigue on the top of the stem at the weld...

Why would you expect that?

EoinC said:

Why would you expect that?

the top part would be under tensile stress and the bottom under compresive stress. cracks expand, propagate, faster under tensile stress because the material is being pulled apart, decreasing the surface area. this means that the same amount of force is divided among the smaller area so it allows the crack to grow larger. when under compression the surface area doesnt decrease as much because the parts are being pushed together so the broken area can still transfer a partial load. this means that cracks are slower to develop and are less likely to cause failure. and the failure would occur at the weld because of stress concentrations present in curved surfaces and joints. the greater the radius, the smaller the stress generated at that point.

spacefuzz said:

the top part would be under tensile stress and the bottom under compresive stress...

Handlebar stems are under the highest tensile stress on the bottom and sides as the greatest force placed upon them is when the rider is pulling hard back / up on them from one or both sides. Their downward load-carrying role (supporting the weight of the rider that is not supported by the saddle / pedal combination) is minimal.
If a handlebar stem breaks during a sprint (when the greatest forces are placed upon it), the rider will go [censored] over [censored] backwards off the bike. The area under compression (apart from twisting moments) when the greatest loads are applied is around the top of the stem.

EoinC said:

Handlebar stems are under the highest tensile stress on the bottom and sides as the greatest force placed upon them is when the rider is pulling hard back / up on them from one or both sides. Their downward load-carrying role (supporting the weight of the rider that is not supported by the saddle / pedal combination) is minimal.
If a handlebar stem breaks during a sprint (when the greatest forces are placed upon it), the rider will go [censored] over [censored] backwards off the bike. The area under compression (apart from twisting moments) when the greatest loads are applied is around the top of the stem.

but when you pull up on one side you are usually pushing down on the other creating a torque on the stem.

spacefuzz said:

but when you pull up on one side you are usually pushing down on the other creating a torque on the stem.

Assuming that you were pushing and pulling with exactly the same force. The shape of the stem comes into play too. Because of the angle of the stem, there's a sharper corner at the edge of the weld bead on the bottom than there is on the top.

artmichalek said:

Assuming that you were pushing and pulling with exactly the same force. The shape of the stem comes into play too. Because of the angle of the stem, there's a sharper corner at the edge of the weld bead on the bottom than there is on the top.

a good point. i guess well never know unless peter decides to send us the stem. always fun to wonder why things break though.

spacefuzz said:

but when you pull up on one side you are usually pushing down on the other creating a torque on the stem.

Maybe you are, but I am not. In a sprint, I am pulling back and up with both arms, bracing against the pedals. The tortional loading comes from one arm pulling harder than the other depending upon which pedal is receiving forward / downward load.
I have broken handlebar stems (twice) and handlebars (twice) - tell me how fortunate I am. On all 4 occasions, the movement has been upward and back. The only way you could break a standard handlebar stem across the top would be by putting a 'large' force downwards on it. The only time I could see this happening would be during a crash (and then it's unlikely as your arms would fold at the elbows), or when dubbing your mate on the handlebars while he / she is eating hi / her 4th quarter-pounder for the day.

spacefuzz said:

a good point. i guess well never know unless peter decides to send us the stem. always fun to wonder why things break though.

Thanks to everybody who has replied to this thread. If you'd like to take a look at the broken stem I'd be glad to send it to you just because I'm curious why it happened as well. I was probably just going to throw it away cause I bought the bike used (no evidence of crash damage) and can't vouch for the prior owners treatment (in another words I have not a leg to stand on with Cannondale in terms of a replacement of the part)

Peter Verdesi said:

...(in another words I have not a leg to stand on with Cannondale in terms of a replacement of the part)

Peter,
I would actually be sending it to Cannondale for a different reason. While they may or may not replace it for you, they could at least have an inspection carried out on it which may point to an avoidable manufacturing flaw, allowing them to prevent someone else from eating ashphalt.

EoinC said:

Maybe you are, but I am not. In a sprint, I am pulling back and up with both arms, bracing against the pedals. The tortional loading comes from one arm pulling harder than the other depending upon which pedal is receiving forward / downward load.
I have broken handlebar stems (twice) and handlebars (twice) - tell me how fortunate I am. On all 4 occasions, the movement has been upward and back. The only way you could break a standard handlebar stem across the top would be by putting a 'large' force downwards on it. The only time I could see this happening would be during a crash (and then it's unlikely as your arms would fold at the elbows), or when dubbing your mate on the handlebars while he / she is eating hi / her 4th quarter-pounder for the day.

He mentioned climbing, not sprinting. when out of the saddle climbing hard I rock the bike by pulling up with one hand slightly and pushing down with the other slightly. If I am sitting and climbing hard I tend to slide back in the saddle and pull on the bars putting the stem under more of a compressive load rather than a bending moment.

spacefuzz said:

He mentioned climbing, not sprinting. when out of the saddle climbing hard I rock the bike by pulling up with one hand slightly and pushing down with the other slightly. If I am sitting and climbing hard I tend to slide back in the saddle and pull on the bars putting the stem under more of a compressive load rather than a bending moment.

You expected to see the stem crack along the top. this would require a downward force. In your riding, how much of a downward component do you believe you are placing on the stem? The most that is possible is a part of the upper body's weight.
There may be a light supporting (downward) component during normal flat riding (depending upon your position on the bike), but when the greatest forces come into play (sprinting and climbing), those forces will be upwards / backwards, along with the torsional side loading which induces tearing.
Pulling on the handlebars allows you to transmit a greater portion of the body's power to the pedals. If this were not the case, you could climb just as fast riding no-hands. If you look at photo's of a decent rider climbing in the saddle, you will see that the tendons in the top of the forearms are raised and prominent, meaning that the arms are pulling and lifting. Note that the elbows are higher than the handlebar stem. Any downward force will be minimal.
I think that, if you analyse your riding next time you try to climb a hill, you will find that the rocking comes from pulling up on the bar (on the same side as the pedal you are loading), not from pushing down on the other side.

Most headstems of that style have cracks that propogate from the bottom. When I first noticed the cracks on one of my old head stems I drilled out the cracks on both sides until I got a chance to replace it. I also had a look at the headstem of every bike with that style of headstem that I came across. Most had cracks of around 10mm and some had cracks almost an inch long. For this reason my lbs refuses to sell bikes with this style of headstem or bikes that have that style of headstem.

EoinC said:

You expected to see the stem crack along the top. this would require a downward force. In your riding, how much of a downward component do you believe you are placing on the stem? The most that is possible is a part of the upper body's weight.
There may be a light supporting (downward) component during normal flat riding (depending upon your position on the bike), but when the greatest forces come into play (sprinting and climbing), those forces will be upwards / backwards, along with the torsional side loading which induces tearing.
Pulling on the handlebars allows you to transmit a greater portion of the body's power to the pedals. If this were not the case, you could climb just as fast riding no-hands. If you look at photo's of a decent rider climbing in the saddle, you will see that the tendons in the top of the forearms are raised and prominent, meaning that the arms are pulling and lifting. Note that the elbows are higher than the handlebar stem. Any downward force will be minimal.
I think that, if you analyse your riding next time you try to climb a hill, you will find that the rocking comes from pulling up on the bar (on the same side as the pedal you are loading), not from pushing down on the other side.

I said I pulled up and pushed down. so why are you arguing.

EoinC said:

Peter,
I would actually be sending it to Cannondale for a different reason. While they may or may not replace it for you, they could at least have an inspection carried out on it which may point to an avoidable manufacturing flaw, allowing them to prevent someone else from eating ashphalt.

Excellent discussion guys,but I'd just like to join Eoinc in urgeing you to send the stem back to Cannondale for their examination an analysis. They may have a production /material problem. I'll bet that they replace it if they do, since you took the time and trouble to submit it.